The general aim of the planned work is to provide a better understanding of neural mechanisms associated with nociception and pain. The receptors (primary afferent units) responsible for pain are one particular target. The experiments will deal with their identification, their characteristics, the processes leading to their excitation and the latter's relation to changes in their sensitivity. A second aspect of the work proposes to further examine the part in central mechanisms of a recently established nociceptor and thermoreceptor projection to the posteromarginal cells of the dorsal horn (Rexed's lamina I). A third objective is to study the functional organization of thalamic regions receiving terminations from first order (receptor) and Lamina I neurons. The work is to be done on rhesus monkeys and cats. It will employ both electrophysiological recordings from single neurons and morphological techniques to localize recording sites. The physiological work will depend upon the selective use of quantitatively controlled "natural" stimulation (a) to analyze events related to noxious excitation, and (b) to determine the contribution of different primary afferent units to central nervous system activity.